The invention relates to an image projection system comprising an illumination system with a light source for supplying an illumination beam, a modulaton system with at least one reflecting image display panel for modulating this light beam in conformity with image information to be projected, and a projection lens system for projecting said image information, a color-separating system being arranged between the modulation system and the illumination system, and a polarizing beam splitter being arranged between the illumination system and the color-separating system.
An image projection system of the type described in the opening paragraph is known from, for example European patent application EP 0 734 184. The system described in this application comprises an illumination unit with a light source for supplying an illumination beam. This unpolarized illumination beam is incident on a polarizing beam splitter (PBS) which ensures that one of the two complementary linearly polarized beam components, for example the s-component, is removed from the light path. The remaining component, the p-component, is split up by a color-separating system into three sub-beams each having a different wavelength. Each sub-beam is incident on a suitable reflective display panel. The display panel used in this case is a liquid crystalline display panel operating on the basis of polarization rotation of incident light. The display panel is subdivided into pixels which, in conformity with the image to be projected, are energized or not energized. An energized pixel will rotate the direction of polarization of incident light, while an unenergized pixel will leave the direction of polarization unchanged. Moreover, a xcex/4 plate is arranged between the projection lens and the polarizing beam splitter for reducing the amount of light reflected from the optical system to the screen. Light passing through the xcex/4 plate goes to the lens where it is reflected and will pass through the xcex/4 plate again. Since this light passes through the xcex/4 plate twice, it will undergo a polarization rotation of 90xc2x0 so that this light will be removed from the light path upon its arrival at the PBS.
A display panel as described hereinbefore is generally provided with a linear polarizer on the illumination side. A pixel which is unenergized will reflect incident light specularly without changing the direction of polarization. The reflected light is thus again passed by the polarizer. Such a pixel will be observed as a bright pixel. A pixel which is energized will change the direction of polarization upon specular reflection. Consequently, this light will no longer be passed by the polarizer. Such a pixel will thus be observed as a dark pixel.
A drawback of the image projection system described in said European patent application is that unwanted polarization rotation will occur in the color-separating system, causing a loss upon polarization separation in the PBS. As a result, the dark state will degrade and the contrast to be realized is limited.
It is an object of the present invention to provide an image projection system in which the above-mentioned drawback is obviated and a relatively high contrast is realized.
To this end, the image projection system according to the invention is characterized in that the reflective display panel is a liquid crystalline display panel of the diffusing type, and a xcex/4 plate is arranged between the reflective portions of the pixels of the display panel and the polarizing beam splitter.
A xcex/4 plate is to be understood to mean an element which converts a linearly polarized beam into a circularly polarized beam, or conversely. The xcex/4 plate preferably has a broad band so that it is active in the full visible wavelength range. A broad-band xcex/4 plate is a transparent element composed of, for example a plurality of layers and realizing such a phase shift in a beam at all wavelengths (xcex) in the visible wavelength range that circularly polarized radiation is converted into linearly polarized radiation. Such a xcex/4 plate is described in, for example the publication: xe2x80x9cRetardation Film for STN-LCDs xe2x80x98NRFxe2x80x99xe2x80x9d by the firm of Nitto Denko in SID ""92 Exhibit Guide, Society for Information Display, May 17-22, 1992, Boston, Mass., USA.
The proposal to use display panels of the diffusing type is based on the inventive idea that, save for a small fraction, the light is incident outside the acceptance angle of the projection lens due to the diffuse reflection which occurs on pixels in the diffusing state, and consequently such a pixel has a relatively good dark state. By combining a diffusing display panel with a xcex/4 plate, the detrimental effect on the dark state due to the unwanted polarization rotation occurring in the color-separating system is reduced considerably, while the quality of the bright state is maintained.
In a liquid crystalline display panel, the pixels may be switched between the transparent and the diffuse state. If the display panel comprises, for example a PDLC (polymer dispersed liquid crystal), the pixel will be in the diffuse state if no voltage is applied across the relevant pixel, and in the transparent state if a voltage is applied across this pixel.
In the transparent state, incident polarized light will be specularly reflected on the reflecting portion of the relevant pixel of the display panel, whereas in the diffusing state the light will be diffusely reflected and also depolarized. Due to this reflection, the diffuse, depolarized light will reach the polarizing beam splitter where approximately one half will propagate towards the illumination system and the other half will propagate towards the projection lens system. Since the angular distribution of the light has been changed completely as a result of the diffusion, only a very small fraction will be incident within the acceptance angle of the projection lens so that the pixel will be observed in the dark state.
By arranging a xcex/4 plate between the polarizing beam splitter and the reflecting portions of the pixels of the display panel, it is ensured that the direction of polarization of the specularly reflected light is rotated through 90xc2x0, while the xcex/4 plate has no effect on the diffuse, depolarized light.
In this way, an improved dark state is realized, while maintaining the quality of the bright state. This means that a relatively high contrast can be realized.
A further advantage of this embodiment is that a compact configuration is possible because in this case no off-axis optical system is required for the diffusing display panel. In fact, since the on-going and the projected beam generally have the same direction of polarization when using a reflecting diffusing display panel, these beams are to be spatially separated. In the present invention, these beams are polarized and this requirement of spatial separation may be dispensed with so that the system can be made more compact.
Moreover, it is sufficient to use optical components in the system according to the invention, for which the requirement as regards stress-related birefringence is relatively lenient. In systems, in which the operation of display panels is based on polarization rotation of incident light, such as, for example TN-LCDs, the requirements imposed on the stresses allowed in the substrates are very severe because they have a detrimental influence on the direction of polarization. Optical components having a low stress-related birefringence are, however, relatively expensive.
A further embodiment of the image projection system according to the invention is characterized in that the xcex/4 plate is arranged at an outer side of the display panel facing the illumination system.
The xcex/4 plate may be arranged on the display panel but may be alternatively arranged in a separate position. The linearly polarized light from the beam splitter will be converted by the xcex/4 plate into circularly polarized light. In the case of specular reflection, which is the case with a transparent pixel, the circular direction of polarization will be inverted upon is reflection. Upon a second passage through the xcex/4 plate, the circularly polarized light will be converted into linearly polarized light again. The direction of polarization of this light will be rotated through 90xc2x0 with respect to the original light from the beam splitter. Consequently, upon arrival at the beam splitter, the reflected light will be passed by the beam splitter towards the projection lens system.
The diffusely reflected and depolarized light will be split up into two components at the beam splitter. One component will be sent towards the light source and only a very small fraction of the other component reaching the projection lens system will fit within the acceptance angle of the projection lens.
A preferred embodiment of the image projection system according to the invention is characterized in that the xcex/4 plate is integrated in the display panel and is arranged between the layer of liquid crystalline material and the reflecting portions of the pixels.
The conversion from linearly polarized light to circularly polarized light is now effected within the display panel. This has the advantage that parasitic reflections on the substrate and the electrodes of the display panel can be suppressed. The parasitic reflections on the substrate and the electrodes have the same direction of polarization as the incident light from the light source and will consequently be sent towards the light source by the PBS so that the dark state will not be degraded by these reflections.
A further embodiment of the image projection system according to the invention is characterized in that a polarization conversion system is arranged between the light source and the polarizing beam splitter.
Consequently, substantially all of the light supplied by the light source is converted into light having the same direction of polarization, rather than that approximately half the light supplied by the light source is deflected from the light path. The light output of the illumination system is thereby increased considerably.
A further embodiment of the image projection system according to the invention is characterized in that the illumination system comprises an integrator system.
The integrator system ensures that the beam supplied by the illumination system is a homogeneous light beam. Known embodiments of an integrator system are, for example an optically transparent bar, an optical tunnel with reflective walls or a combination of two lens plates.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.